The present invention relates to a high frequency switch circuit device formed on a semiconductor substance for gating, i.e., selectively transmitting or blocking, a high frequency signal passing therethrough.
In recent years, there have been drastic developments in the art of mobile communication devices such as portable telephones or personal digital assistants (PDAS). In order to meet the demands for increasing the functionality of such devices and for reducing the size and weight thereof, attempts have been made in the art to reduce the size and the power consumption of, and to increase the degree of integration of, semiconductor integrated circuits at a high pace.
In a mobile communication device, a high frequency switch circuit for gating a high frequency signal is an important circuit that is used for switching between a transmission mode and a reception mode of an antenna circuit. As a switching element for gating a high frequency signal, those using a GaAs-FET provided on a gallium arsenic (GaAs) substrate are widely known in the prior art. Along with the recent developments in the minute processing techniques, an Si-FET provided on a silicon substrate has also been used as a high frequency signal switching element. As compared to a GaAs-FET, an Si-FET can be manufactured at a lower cost since a silicon substrate itself is inexpensive and the mass production techniques therefor have been well established. Where a gallium arsenic substrate is used, it is typical that devices which are not required to have high frequency characteristics, or those which are required to have high frequency characteristics but not high-level frequency characteristics, are separately provided on a silicon substrate in order to reduce the chip area of the gallium arsenic substrate as much as possible since a gallium arsenic substrate is expensive. In contrast, where a silicon substrate is used, there is an advantage that it is possible to provide an integrated circuit device by integrating elements (active elements and passive elements) that are required for a mobile communication device together with a high frequency signal switching element into a single substrate as much as possible.
In such a case, unlike a GaAs-FET using a semi-insulative substrate, an Si-FET using a silicon substrate, which functions as a conductor, is a 4-terminal element that requires a back gate (corresponding to a well region under a channel in the substrate) for fixing a substrate potential, as well as a source, a drain and a gate. Therefore, in an Si-FET, a high frequency signal is likely to leak from the drain or the source to the back gate via a capacitance between the drain and the back gate and a capacitance between the source and the back gate. Thus, the transmission loss of an Si-FET may increase, thereby deteriorating its performance as a switching element. In order to avoid such a problem, attempts have been made in the prior art to reduce the leak of a high frequency signal from the back gate of an Si-FET.
FIG. 8 is an electric circuit diagram of a high frequency switch circuit described in Japanese Laid-Open Patent Publication No. 10-242826, illustrating an example of a conventional high frequency switch circuit. As illustrated in the figure, a conventional high frequency switch circuit is provided between a first node P1 and a second node P2. The high frequency switch circuit includes: a first transistor (FET) 201 and a second transistor (FET) 202 serially connected to each other; a third transistor (FET) 203 provided between the first node P1 and the ground; a fourth transistor (FET) 204 provided between the second node P2 and the ground; resistors 205 to 208 provided so as to be connected to the gates of the transistors 201 to 204, respectively; a resistor 209 provided between the back gate (the node denoted by “BG” in FIG. 8) of each of the first and second transistors 201 and 202 and the ground; a resistor 210 provided between the back gate of the transistor 203 and the ground; and a resistor 211 provided between the back gate of the transistor 204 and the ground. Control voltages Vc and /Vc are used to switch the connection between a third node P3 (connected to the node between the first and second transistors 201 and 202) and the first node P1 and the connection between the third node P3 and the second node P2 from one to another. Specifically, the signal transmission path is switched between the path between the first node P1 and the third node P3 and the path between the second node P2 and the third node P3 by using the control voltage Vc to turn ON/OFF the first and fourth transistors 201 and 204 and the control voltage /Vc to control the gate voltage of the second and third transistors 202 and 203 so as to turn ON/OFF the respective transistors.
In the conventional high frequency switch circuit, the resistors 205 to 208 are provided between the ground and the back gates of the transistors 201 to 204, respectively, whereby it is possible to reduce the transmission loss of a high frequency signal due to a drain-back gate capacitance or a source-back gate capacitance.
The conventional high frequency switch circuit described above is, in principle, capable of reducing the transmission loss of a high frequency signal due to a drain-back gate capacitance or a source-back gate capacitance. However, in an integrated circuit device including the high frequency switch circuit, the transmission loss of a high frequency signal in the high frequency switch circuit may increase, thereby deteriorating its performance as a high frequency switch, for the following reason.
Where a plurality of circuits are integrated together into a single silicon substrate, the silicon substrate needs to be grounded in order to prevent the parasitic oscillation of each transistor and to separate the circuits from one another. As a result, in the high frequency signal switching element described in the above publication as illustrated in FIG. 8, the back gate connected to the ground via a resistor is connected to the ground also via a path through the silicon substrate. Therefore, a high frequency signal leaks from the back gate to the ground via the silicon substrate, thereby increasing the transmission loss.